Magnetic random access memories (MRAMs) are expected as nonvolatile random access memories (RAMs) capable of high-speed operations, and have been actively studied and developed.
The fundamental structure of a magnetic tunnel junction (MTJ) element that is a memory element of an MRAM is formed with three layers, which are a magnetic layer, an insulating layer, and another magnetic layer. One of the magnetic layers is called the storage layer, and the other one of the magnetic layers is called the reference layer. The intermediate insulating layer is called the tunnel barrier, and is formed with an insulator that is very thin but can allow a tunneling current to flow. The magnetization direction of the reference layer does not change before and after a write current is applied thereto. The magnetization direction of the storage layer is changeable, and may be in a parallel state or in an antiparallel state with respect to the magnetization direction of the reference layer. When the magnetization directions of the storage layer and the reference layer are parallel to each other, the electrical resistance between the storage layer and the reference layer via the tunnel barrier is low, because of a magnetoresistive effect. When the magnetization directions of the storage layer and the reference layer are antiparallel to each other, the electrical resistance is high, because of a magnetoresistive effect.
A known method of causing a magnetization switching (writing) in the storage layer of an MTJ element uses a spin Hall effect or a spin-orbit interaction (spin-orbit coupling). A spin-orbit interaction is a phenomenon in which electric current is applied to a conductive nonmagnetic layer so that electrons having spin angular momenta (hereinafter also referred to simply as the spin) of the opposite orientations from each other are scattered in the opposite directions, and a spin current is generated. There are known MRAMs of a SOT (Spin Orbit Torque) type in which a MTJ element is stacked on a nonmagnetic layer having a large spin-orbit interaction, and the magnetization direction of the storage layer of the MTJ element is switched by SOT. However, in an MRAM of this type, two transistors are required for one MTJ element in each memory cell, and therefore, miniaturization of the memory cells is difficult to achieve.
Meanwhile, there also are known MRAMs in which voltage is applied to an MTJ element, and a magnetization switching is caused by virtue of a voltage-controlled magnetic anisotropy (VCMA) effect that changes the anisotropy energy of magnetization. A cell structure of an MRAM of this VCMA type can be formed with one MTJ element and one corresponding transistor. However, to control the processional movement of magnetization by adjusting the pulse width of the write pulse in write principles, extremely accurate pulse width control on the order of picoseconds needs to be performed, and RC delay in wiring lines due to an increased capacity presents another problem.
To solve these problems, a spintronics memory has been suggested. The spintronics memory performs writing collectively on MTJ elements (equivalent to eight bits, for example), using both the SOT method and the VCMA method. Such a spintronics memory has a structure in which the MTJ elements are stacked on a nonmagnetic layer made of a conductive material having a large spin-orbit interaction.
When writing is performed in the structure of this spintronics memory, a potential is applied to the reference layer of the MTJ element, and a write current is applied to the nonmagnetic layer of the MTJ element. At the time of writing, the write current does not pass through the tunnel barrier in the MTJ element. Accordingly, the reliability of the tunnel barrier becomes higher, and read disturb is alleviated as the current paths of the read current and the write current differ from each other, as in an MRAM of the SOT type. Furthermore, the spintronics memory uses VCMA to adjust the energy barrier. Thus, the required degree of pulse width accuracy, which presents a problem in a magnetization switching by VCMA, is lowered.
In the spintronics memory, however, the write window at a time of writing, or the difference between the high-resistance value and the low-resistance value to be written into an MTJ element, becomes narrower, as will be described later.
Also, the read window at a time of reading, or the difference between the high-resistance value and the low-resistance value to be read from an MTJ element, becomes smaller.